The Study On Preparation Of Hematite Nano-particles For Controlled-size And Dispersive Techniques

Nano-particles of ferric oxides may contain unusual forms of structural disorder that can substantially modify materials and thus can not solely be considered as small pieces of bulk material. Therefore, it is an important chemical material in industry that can be applied extensively in many fields. And nano-dispersed α-Fe₂O₃ particles have been widely applied in fine ceramics, catalysis, light filter, light absorption, medicine, anticorrosion, pigment, photosensitive materials and magnetic recording media, etc. Today efforts are continuing to produce nano-dispersed α-Fe₂O₃ particles, uniform in shape, size and composition as ideal constituent for varieties of advanced materials. Various processing techniques developed for the production of such particles are spray roasting, such as hydrothermal processes, sol-gel method, spray pyrolysis method and so on. These methods are effective preparation of ferric oxide particles, but these techniques have the disadvantages of requiring very complicated skills, strict conditions and high costs, which greatly limit their application in industry.In our laboratory, a new method of preparing nano-sized hematite by catalytic phase transformation in solution was successfully developed. In this method, FeCl₃, Fe(NO₃)3 or Fe₂(SO₄)₃ are main raw materials and ferrihydrite gel is firstly prepared, then α-Fe₂O₃ is obtained by the catalytic liquid phase transformation of ferrihydrite in aqueous solution under boiling reflux condition. It was considered that the novel method has such outstanding characteristics as simple equipment, cheap and easily got raw materials, high initial concentration, gentle reaction condition, fast rate of phase transformation, gentle reaction conditions etc.Since no systematic information is available on the micro-structural and properties of hematite particles, and the phase transformation process is very complicated and influenced by many factors such as the concentration of iron salt, the type of anion, pH, temperature and time of crystallization and so on, it is far away from the application in large-scale at the moment, but the recent work has been undertaken to generate such information. In this paper, this precipitation was further investigated, in order to prepare different batches of nano-sized hematite particles of varying sizes. The results were explained by dissolution/re-precipitation and solid-transformation on the basis of electron transfer. And at the same time the theoryconsummated with present experiments.On the basis of the former work of our laboratory, the main research aspects are asfollows in this paper:(1) Effects of the variations in three levels of raw materials, Fe3+ concentration, the ratio of [Fe2+]/[Fe3+], temperature and pH of ferrihydrite preparation was studied by simply orthogonal testing design. According to the layout of 3 levels with 4 or 3 factors , 9 different experiments were done for pure hematite and the size of nano-particle. The results show that the effects of different raw materials and range about [Fe2+]/[Fe3+] are main for pure hematite, and the effect on size of hematite particles is Fe3+ concentration > temperature of ferrihydrite preparation > pH of ferrihydrite formation. An analysis of the margin of experimental data, showed that the concentration of ferric iron is relative importance to the size of hematite particle. Consequently, The optimum reaction conditions were determined, Fe(NO)3 or FeCU as raw material, [Fe2+]/[Fe3+]=0.02, formation of ferrihydrite at pH=9 and temperature=40°C. In these conditions, 50+1 Onm hematite spherical particles were prepared.(2) All kinds of facts that may affect the purity and particle-size of hematite prepared by the catalytic phase transformation method were studied. The TEM(transmission electron microscopy), FT-IR(fourier transform infared spectroscopy), XRD(x-ray diffraction), ED(electron diffraction) were used to investigate the course during phase transformation, the morphology and the sizes of hematite. The mechanism of dissolution/re-precipitation and solid transformation were applied on the explanation for the experimental results. At the same time the mechanisms were improved.The result shows that pH value decreases when the ferrihydrite prepared at pH=7 and increases when the ferrihydrite prepared at pH=9 with increasing of reactive time. The finished time of phase transformation prolongs with decreasing of Fe3+ concentration. Fe3+ concentration is the main factor that affects the dimensions in size of hematite particles. The changes were different at different pH when the ferrihydrite gel was formed. And others various facts that may affect the size were also studied. In addition, it was found that initial pH and temperature have significantly influenced on the intermediates forms of ferrihydrite gel. In short, the present results show that hematite particles of varying sizes and shapes can be prepared by catalytic phase transformation through control of certain conditions. Two scopes in size of hematite nano-particles were successfully obtained, one is about 70nm, the other is about 40nm. So the size of hematite nano-particles can be controlled by the control of reaction conditions. This method provides the experimental grounds for the industrial production.